Influenza viruses, particularly types A and B viruses, have been associated with major outbreaks of influenza, and are a serious cause of morbidity and mortality of animals. Infection with type B influenza is usually milder than type A. Influenza A viruses are hosted by birds, but may infect several species of mammals. All known subtypes of influenza A are endemic in birds. Birds have the greatest number and range of influenza strains. The influenza virus strains have been and are undergoing antigenic drift, probably driven by selective antibody pressure in the populations of humans infected, leading to seasonal epidemics in several geographical regions. Recently, it was found that highly pathogenic avian influenza A viruses of the H5N1 subtype were circulating in eastern Asia. Some strains of the human H5N1 viruses were isolated and studied. In 2005, some H5N1 isolates lacking the polybasic cleavage site in the hemagglutinin gene were produced by reverse genetics in anticipation of the possible need to vaccinate humans. See Emerging Infectious Disease (2005) 11(10):1515-1521.
An influenza virus is an RNA virus of the Orthomyxoviridae family and comprises a segment negative RNA genome, which codes for about 10 influenza proteins. The 10 influenza proteins include RNA-directed RNA polymerase proteins (PB2, PB1 and PA), nucleoprotein (NP), neuraminidase (NA), hemagglutinin (HA, which after enzymatic cleavage is made up of the association of subunits HA1 and HA2), the matrix proteins (M1 and M2) and the non-structural proteins (NS1 and NS2) (Krug et al. (1989) in The Influenza Viruses (R. M. Krug, ed.), Plenum Press (New York) pages 89-152).
For the past several decades, fertilized chicken eggs have been used as a host system to replicate influenza viruses for use in vaccine production. African green monkey kidney (Vero) cells and Madin-Darby canine kidney (MDCK) cells have been widely used for manufacturing seasonal influenza vaccines (Audrey et al. (2004) Expert Opin. Biol. Ther. 4(5):709-17 and Ghendon et al. (2005) Vaccine 23(38):4678-84).
A current influenza H5N1 vaccine strain, NIBRG-14, was provided by the National Institute of Biological and Serum Control, the United of Kingdom (UK NIBSC), which is a reassortant virus containing HA and NA gene segments of A/Vietnam/1194/2004 (H5N1) virus and the other six internal gene segments of A/PuertoRico/8/1934 (H1N1) virus. It was found that this vaccine strain could grow efficiently in chicken embryonated eggs and MDCK cells but not in Vero cells (Govorkova et al. (1999) Dev. Biol. Stand. 98:39-51). However, in the case of a pandemic influenza, chicken eggs would be insufficient and suboptimal for influenza vaccine production, and there are concerns over the tumorogenic potential of MDCK cells.
Therefore, there is a need for an influenza virus strain that is able to grow efficiently in Vero cells and suitable for increased production of an influenza vaccine from a mammalian host cell. The present invention satisfies this need.